In machines which are used for manufacturing and inspecting semiconductor components, objects must often be precisely positioned. For example, it may be necessary to position wafers extremely precisely below a tool of an exposure or inspection unit. In that case, the wafer lies on a table that is movable in six degrees of freedom and is moved via an associated drive. Thus, the table acts as an object whose position is to be determined with great accuracy. In order to position the table via the drive and an associated control unit, it is necessary to generate position signals with respect to the spatial position of the table with the aid of extremely precise position-measuring devices.
Interferometers and grating-based optical position-measuring devices are usually used as extremely precise position-measuring devices in such machines. However, problems result in both types of position-measuring devices when the travel path of the table along an intended axis of motion is greater than the extension or length of the table in this axis of motion.
With the aid of interferometers, the position of the table in a travel direction or axis of motion is usually determined by the probing of a measuring reflector in the form of a mirror using a measuring beam. In so doing, the measuring beam travels parallel to the determined axis of motion and is anchored so as to be fixed in position with the machine. If the table additionally moves in a second travel direction, it must be ensured that the measuring beam strikes the mirror at each position of the table. If the distance traveled in the second travel direction is greater than the side length of the table, and therefore also greater than the side length of the mirror mounted on the table, then in certain positions, the measuring beam no longer strikes the mirror, and the interferometer can no longer generate correct position signals, that is, the interferometer loses the determined position. To avoid such a loss of the position, the mirror must therefore be scanned with the measuring beam of an additional second interferometer axis that is offset in the second travel direction relative to the first measuring beam, such that at each position of the table, at least one of the two measuring beams strikes the mirror. Since interferometers usually only measure a position incrementally and not absolutely, it is furthermore necessary that the absolute position be transferred between both interferometer axes, before one of the measuring beams no longer strikes the mirror. This is referred to as a position transfer. Therefore, using interferometers which measure the table position from outside, i.e., from a fixed reference system, without further auxiliary axes and position transfer, only a maximal travel path which corresponds to the length of the table along the respective axis of motion is measurable.
If, as an alternative, the corresponding interferometer components are carried along on the moving table and the interferometer measures outwards in the direction of the fixed reference system, problems then result in connection with the light feed and the detection of the interferometer signals. Furthermore, due to the additional interferometer components on the table, the mass of the moving table increases, which impairs its dynamic performance.
In addition, a large measuring loop results as a further disadvantage. This means that the interferometer does not determine the position of the table directly relative to the respective tool, but rather relative to a remote reference, usually the mounting location of an optical unit of the interferometer, i.e., the interferometer head. Consequently, a possible drift in position between the optical unit and the tool is reflected directly in a drift of the measured position. Typically, the spatial distance between the optical unit and the tool may lie in the range of 1 to 2 m, whereas the measuring accuracies required for such applications lie in the nm-range.
Furthermore, in order to use interferometers to determine the motion of the table along two axes of motion perpendicular to one another, it is necessary that the moving table be accessible optically from two sides, which in turn results in certain restrictions in the design of the respective machine.
In the event grating-based position-measuring devices, made up of a measuring standard and one or more scanning units, are used, the measuring length available is limited by the length of the specific measuring standard. For a highly dynamic application, it is considered advantageous as a matter of principle to provide the scanning unit of such a position-measuring device in the fixed reference system, and the measuring standard on the moving table. On the other hand, however, the table should be constructed as compactly as possible, which in turn restricts the possible length of the measuring standard considerably. In particular, it is considered to be disadvantageous to choose a length for the measuring standard that is greater than that of the moving table along the axis of motion. Thus, the realization of travel paths, especially in the applications mentioned above, may be problematic with such an arrangement of conventional grating-based position-measuring devices as well, if the requisite travel path is greater than the length of the moving table along this axis of motion.
On the other hand, if, conversely, the scanning units of the grating-based position-measuring device are mounted on the moving table, then its mass in turn increases and influences its dynamic performance disadvantageously. The necessary cable connections which connect the scanning units to the fixed reference system are also considered to be disadvantageous in this case.
Conventional systems are described, for example, in European Published Patent Application No. 1 469 351, U.S. Pat. No. 7,907,287, U.S. Pat. No. 7,751,060 B2, and the presentation by Gerd Jaeger, Doctor of Engineering at the third ITG/GMA professional convention “Sensoren und Messsysteme” (Sensors and Measuring Systems) (Mar. 9-11, 1998, Bad Nauheim) having the title “Laserinterferometrische Messverfahren—Moeglichkeiten, Grenzen und Anwendungen” (Laser-interferometric Measuring Methods—Possibilities, Limits and Practical Applications).